Lithium sulphate ultrasonic transducer



SR LU-v i amamma ,iggg'" SEARCH Room DIXON ETAL LITHIUM SULPHATE ULTRASONIC TRANSDUCER Filed March 27, 1968 Q ff2 Inventors Nor/77012 E. Dz'xazz lazllzam Jwlema/z i tar/ray 3546012 OR IN 427/10Q United States LITHIUM SULPHATE ULTRASONIC TRANSDUCER Norman E. Dixon and William J. Coleman, Pasco, Wash.,

assignors to the United States of America as represented by the United States Atomic Energy CommlSSlOIl Filed Mar. 27, 1968, Ser. No. 716,592 Int. Cl. B44d 1/20 U.S. (:1. 117-413 7 Claims ABSTRACT OF THE DISCLOSURE CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to an ultrasonic transducer and particularly to a process for making a high-frequency lithium sulphate transducer.

Lithium sulphate ultrasonic transducers have a variety of uses and are specifically applicable to nondestructive testing of nuclear fuel elements. If the test specimen is thin or the defects small or if accurate time or phase information is needed, the transducer characteristics often determine not only the quality but also the feasibility of the test. Transducers having improved sensitivity and higher frequencies are required for many of the applications now coming into use.

The fundamental frequency of a transducer is related to the crystalthickness and surface finish and, generally, the thinner the crystal and the smoother the surface finish, the higher the fundamental frequency. A transducer produced by the process of this invention has greater sensitivity and higher fundamental frequency than lithium sulphate transducers heretofore available. While lithium sulphate transducer's'having frequencies of; about 28 megahertz (mHz.) are commercially available, transducers having frequencies over 55 mHz. have been produced by the process of this invention, and there is no reason to believe that frequencies of over 100 mHz. cannot be attached.

The improved lithium sulphate transducer of the present invention is produced by a unique combination of steps, the first of which involves etching lithium sulphate crystals with water, and the second and third steps of which involve vacuum drying the lithium sulphate crystals and vapor depositing onto the crystals, under vacuum, various electrode, sealant, and acoustic-impedance matching materials.

BRIEF DESCRIPTION OF THE DRAWING The invention may be better understood by reference to the drawing which is a partially exploded view in cross section of a transducer produced by the process of this invention.

3,546,012 Patented Dec. 8, 1970 DESCRIPTION OF THE PREFERRED EMBODIMENT Transducer 10 has a conducting housing 12, a nonconducting cover plate 14 and a hot terminal 16 extending through the cover plate. A damping material 18 fills housing 12 from the cover plate 14 to the lens assembly 20. Lens assembly 20 has as a first layer a hot electrode 22 connected to hot terminal 16 by a wire 24. The layers of lens assembly 20 which are bonded to each other, in order after the hot electrode 22, are a lithium sulphate crystal 26, a quartz layer 28, a ground electrode 30, a destructive interference lens 32, a water sealant bead 34, and a focusing lens 36. Not all parts of lens assembly 20 as shown are needed for an operable transducer and, in fact, only a pair of electrodes and a piezoelectric ma.- terial are required.

While the lithium sulphate crystals of commercially available transducers are generally finished by optical lapping, the crystal 26 of the present invention was prepared for use in transducer v10 by an entirely different technique. The lithium sulphate crystal26 for transducer 10 was etched to the desired thickness with a solution of 1 part water per 1000 parts alcohol. While many diluents which are nonreactive with lithium sulphate may be used in combination with the water, alcohol is preferred. When both sides of crystal 26 were exposed to the etching solution, the crystal decreased in thickness at a rate of 1 mil per hour, but after the crystal thickness was less than 5 mils, the etching rate increased. Crystal thicknesses between 1 and 1.5 mils were obtained by etching only one side of the crystal. Precise crystal frequency can be measured continuously during etching by capacity coupling a transmitter pulse from electrodes through the etching solution and monitoring, by oscilloscope or other means, the crystal ringing. Since the fundamental frequency of the crystal 26 is affected by the surface finish as well as the thickness, and since the surface finish of the etched crystal is very close to the original finish, the etching process should be started with a crystal having a smooth finish. A transducer with a lithium sulphate crystal 1 mil thick which is smooth at 500 magnification has a fundamental frequency of approximately mI-Iz., which constitutes an improvement by a factor of about 4 over commercially available lithium sulphate transducers.

As stated before, the etching rate increases when the crystal thickness decreases below about 5 mils, and also, at that thickness, handling becomes difiicult. Depositing a backing material, which may be an electrode, a sealant or an impedance-matching material, onbne side of crystal 26 produces a twofold benefit. The'backing material adds strength to crystal 26, thereby eliminating some of the handling difficulties, and also slows the etching rate by sealing one side of the crystal from the etching solution. Adding a backing material to crystal 26 may cause a decrease in the transducer sensitivity because sensitivity is dependent not only on the crystal surface finish and thickness but also on the thickness of any backing material attached to crystal 26 and the quality of the bond between them.

The second and third steps of the process of this invention provide for the deposition of thin layers of a backing material on crystal 26 with an improved bond between the backing material and the crystal. Heretofore electrodes have been attached to lithium sulphate crys tals by gluing metal foils to the crystal. The resultant combination of crystal and electrode has given unsatisfactory results, because the foils are comparatively thick and the cement used to hold them to the crystal generally detracts from the overall transducer-sensitivity. While electron gun vapor deposition of the various electrode, sealant or impedance-matching materials on the transducer crystal has been recognized as desirable, the poor bonds between the crystal and these backing materials resulting from vapor deposition have rendered this technique inapplicable to lithium sulphate crystals.

It was discovered that the inferior bonds were due to the hydroscopic nature of lithium sulphate and that various backing materials could be vapor deposited on lithium sulphate provided that the crystal 26 was first dried under vacuum and the vapor deposition was carried out under vacuum. Heating the crystals to dry them is impractical, because lithium sulphate loses its piezoelectric properties at temperatures of about 120 C. The second and third steps of this invention therefore comprise vacuum drying lithium sulphate crystals and vapor depositing a backing material on the crystals under vacuum. These process steps permit vapor deposition of electrode material, such as silver, aluminum, chromium, copper, etc., or sealant and impedance-matching material, such as quartz, in thicknesses between 1,000 and 10,000 A. and with bonds superior to those heretofore available.

The above process steps were used to vapor deposit electrode material not only on the crystal 26 but also on quartz layer 28. Deposition of a 1,000-3,000 A. layer of quartz on the lithium sulphate crystal prior to etching is desirable because the quartz becomes the primary water sealant for water-immersed lithium sulphate transducers and eliminates the need for an epoxy sealing lens.

The quartz significantly improves the transducer sensitivity, and increases the dielectric strength of the highfrequency elements, thereby allowing higher transmitter voltages to be applied to transducer without punching through or shorting crystal 26. Other materials with lowfrequency attenuation, a high dielectric constant, and similar acoustic impedance to lithium sulphate may be substituted for quartz.

The order in which the different layers of the lens assembly are applied is not of paramount importance as long as the proper process steps are used. For instance, crystal 26 may be partially etched and then vacuum dried to remove all the water from the crystal and stop the etching process. Thereafter, a quartz layer 28 may be vapor deposited under vacuum on the crystal 26 which may then be etched to its final thickness. Clearly, either the quartz layer 28 or the hot electrode 22 may be vapor deposited on crystal 26 as a backing material before any etching takes place, or they may be vapor deposited after crystal 26 has been partially etched.

As stated before, not all of the layers in lens assembly 20 are necessary for an operative device. The number and type will be determined by the end use of transducer 10. Clearly, if transducer 10 is to be used in water, protection for the lithium sulphate will have to be provided. To that end, vapor deposition of a quartz layer 28 or even applying a ground electrode 30 of a silver conductor epoxy could sutficiently protect crystal 26.

It should be noted from the drawing that hot electrode 22 is recessed from the edges of crystal 26. Recessing of hot electrode 22 was accomplished by masking the crystal before deposition. Recessing of electrode 22 eliminates crystal edge corona or voltage breakdown and provides a beam with symmetrical edge effects. It also minimizes collimation effects from the edge of the housing lip when a focused lens 36 is used. An additional advantage is that recessing hot electrode 22 obviates the necessity of insulating the hot electrode from housing 12, thereby reducing the cost of fabrication.

Transducers have been built and tested with lithium sulphate crystals less than about 2 mils thick which correspond approximately to 55 mI-Iz. Both SiO and SiO have been used as water-sealant materials and vapor deposited on crystal 26 in the manner described above. A damping material 18 consisting of a tungsten-loaded epoxy was usually used. No insulation between the damping material 18 and the housing 12 was necessary if the tungsten was allowed to slightly oxidize. It should be noted here that other nonconducting materials may be used instead of tungsten, depending upon the end use of the transducer 10 and the acoustic-impedance matching characteristics of the various layers deposited on th crystal 26.

In some cases, a destructive interference lens 32 was deposited on quartz 28. While the destructive interference lens 32 could have been vapor deposited as was the quartz lens 28, -a different technique was used. A silver conductor epoxy paint was sprayed onto quartz 28 in very thin incremental layers with a spray applicator. By observing an oscilloscope display of crystal ringing from a low voltage-fast rise transmitted pulse, it is quite apparent when optimum destructive interference or damping has occurred. Since 2 to 3 minutes of drying time is required for the paint and the longitudinal velocity changes with drying, the optimum destructive resonance point can be overshot, and as the paint dries the optimum destructive resonance point will repeat. An improvement factor of 8 in the resolution of an air-backed transducer has been observed with this technique. If a silver conductor paint lens is used as described, the ground electrode 30 can be eliminated and the silver conductor paint lens can serve as the ground electrode, the destructive interference lens 32, and a secondary water sealant for crystal 26.

It should be clear that the over-all makeup of lens assembly 20 and the order in which the lens assembly is put together depend upon the end use for transducer 10. A considerable number of materials may be applied to the process of this invention in order to produce a lithium sulphate transducer having superior performance characteristics.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for making a lithium sulfate transducer from a lithium sulfate crystal comprising etching the crystal with water dissolved in a large amount of a diluent, which is nonreactive with lithium sulfate, to a thickness of not less than 5 mils; vacuum drying the crystal; vapor depositing a backing material on one side of the crystal, said backing material comprising an electrode material or an impedance matching material; etching the opposite side of the crystal with water dissolved in a large amount of a diluent, which is nonreactive with lithium sulfate, to a thickness of less than 2 mils; and vapor depositing elec trode material on said opposite side of the crystal.

2. A process according to claim 1 wherein the backing material is quartz and the etching solution is 1 part of water to 1000 parts of alcohol.

3. A process for making a lithium sulfate transducer from a lithium sulfate crystal having a smooth finish comprising vacuum drying the crystal; vapor depositing a backing material under a vacuum on one side of the crystal, said backing material consisting of an electrode material or an impedance matching material; etching the opposite side of the crystal with water dissolved in a large amount of a diluent, which is nonreactive with lithium sulfate, to a thickness of less than 2 mils; vacuum drying the crystal; and vapor depositing under vacuum electrode material on said etched surface of the crystal.

4. The process of claim 3 wherein the etching solution is 1 part of water to 1,000 parts of alcohol.

5. The process of claim 3 wherein the backing material is quartz.

6. The process of claim 5 wherein a ground electrode is vapor deposited under vacuum on the quartz.

7. The process of claim 5 wherein a silver conductor 5 epoxy paint is sprayed onto the quartz which paint serves OTHER REFERENCES as ground electrode and destructive interference lens. Handbook of Chemistry and Physics 42 Chant References Cited cal Rubber Publishing Co., 1961, pp. 598 and 599.

UNITED STATES PATENTS 5 ALFRED L. LEAVITT, Primary Examiner 2,384,500 9/ 1945 $1011 117107-1 c. K. WEIFFENBACH, Assistant Examiner 2,479,540 8/ 1949 Osterberg 117106 2,972,068 2/1961 Hcvwry et al. 3108.2 US. Cl. X.R. 3,378,705 4/1968 Bacon 3l08.2X I 3,427,481 2/1969 Lenahan et a]. 310 8 2 10 1l7l06, 119.6, 215, 217, 227, 229, 15617, BIO-8.2 

